Once it had infected a computer, the software hid in the background and periodically checked free online bulletin boards on a Japanese portal, "livedoor," for commands from its controller. These included instructions to turn keylogging on and off, upload and download files, and post messages to other bulletin boards.
Authorities have discovered multiple versions of the Windows program in the wild, according to reports, and those who have analyzed them say they are complex and unique. Security firm Trend Micro has rated the code's "damage potential" as high, and says users must manually modify their computers' registries to get rid of it.
The complexity of the program, and the meticulous way it was developed, appear to contrast with Katayama's actions in the months when he was allegedly toying with Japan's national police. Authorities have said he occasionally failed to anonymize his Internet activities, and traced him to the online purchase of a small figurine that later appeared in a photo he sent to newspapers.
The FBI cooperated with Japanese police and provided the contents of a Dropbox account used by the suspect, the contents of which included a copy of the "iesys.exe" virus, according to the Nikkei business newspaper.
If Katayama does turn out to be the mastermind behind the program and the one that made threats online, it appears police caught a break in their first public duel with a hacker of his stature. One of their main leads was decidedly low tech -- video footage of him caught by a security camera on Enoshima, the small island where he allegedly planted the memory card on a local cat.
"It appears that he wanted to show off and went too far. If this is true then the police were fortunate he did so, because they probably wouldn't have found him otherwise," said Ochiai.